SPR sensors generate optical signals responsive to the dielectric constant and/or thickness, hereinafter referred to collectively as “optical properties”, of regions of a layer, hereinafter referred to as a “probe layer”, of material contiguous with a thin layer of conducting material. The thin conductive layer, hereinafter an “SPR conductor”, typically has a thickness less than about 100 nm and is generally formed from a metal, usually silver or gold, on a surface of a transparent substrate such as glass. The surface on which the SPR conductor is formed is hereinafter referred to as a “sensor surface”
Optionally, such as in a Kretschmann configuration of an SPR sensor, the sensor surface is a first surface of a prism having a triangular cross section. Light from a suitable light source is directed into the prism through a second surface of the prism so that the light is incident at a non-zero angle of incidence on the SPR conductor from inside the prism. The light is linearly polarized so that it has a “p” component of polarization. The SPR conductor is sufficiently thin so that for angles greater than the critical angle of the light at the interface between the prism and the SPR conductor, the evanescent field of the light extends substantially into the probe layer. Light from the incident light is reflected from interface between the sensor surface and the SPR conductor, exits the prism through a third surface of the prism and is detected by a suitable photosurface, such as for example a CCD.
For a given wavelength of the incident light, there exists a particular angle, hereinafter a “resonance angle”, greater than the critical angle, for which the evanescent field of the p polarization component of the light resonates with a propagation mode of charge density waves of electrons in the SPR conductor. The charge density waves tend to propagate along the surfaces of the SPR conductor and are conventionally referred to as “surface plasmons”. At the resonance angle and angles within an “angular resonance width”, in a neighborhood of the resonance angle, energy is coupled from the evanescent field into surface plasmons.
As a result of energy absorbed from the evanescent field by the surface plasmons, for the given wavelength, reflectance of the light as a function of incident angle decreases substantially for angles within the angular width of the plasmon resonance and exhibits a local minimum at the resonance angle. In addition, phase of reflected light as a function of angle undergoes relatively rapid change for angles within the angular width of the plasmon resonance.
Similarly, for a given incident angle of the incident light, there exists a particular resonance wavelength at which the incident light resonates with a surface plasmon in the SPR conductive layer. Reflectance of the light as a function of wavelength decreases substantially for wavelengths within a “wavelength resonance width” of the surface plasmon and exhibits a local minimum at the resonance wavelength for the given angle of incidence. Phase of reflected light as a function of wavelength undergoes relatively rapid change for wavelengths within the wavelength resonance width.
The SPR resonance angle, resonance wavelength, reflectance and phase changes that characterize a surface plasmon resonance are hereinafter referred to as “SPR parameters”. The SPR parameters are functions of the optical properties of the substrate (e.g. the prism glass), the SPR conductor and, because the evanescent field extends into the probe layer, of the probe layer.
In typical operation of an SPR sensor, generally either the wavelength of light incident on the sensor surface is maintained constant and the incident angle of the light varied or the incident angle is maintained constant and the wavelength varied. Signals generated by the photosurface responsive to the light reflected to the photosurface from a region of the sensor surface under either of these conditions are used to determine a value of at least one SPR parameter for the region. The at least one SPR parameter is used to determine a characteristic of a material, hereinafter a “target material”, that affects the index of refraction of the probe layer by interacting with the probe layer. The target material is generally a liquid or a gas, i.e. a target liquid or target gas, that is transported along a surface of the probe layer by a suitable “flow cell”.
For example, in some applications an SPR parameter is used to identify and assay analytes in a target liquid or gas that flows over the sensor surface of an SPR sensor and interact with components of the probe layer to change at least one the probe layer's optical properties. In some applications an SPR parameter is used to determine a characteristic of an interaction, such as for example an interaction rate, between material in a probe layer and a target material that affects the an optical property of the probe layer. The rate of interaction determines a rate at which the optical property of the probe layer changes and thereby a rate of change of an SPR parameter determined by the SPR sensor. The determined rate of change of the SPR parameter is used to determine the rate of interaction.
SPR sensors and methods are generally very sensitive to changes in an optical property of a probe layer and have proven to be useful in detecting changes in an optical property of a probe layer generated by relatively small stimuli. An SPR probe layer may also be configured as a multianalyte “microarray” that presents on each of a relatively large plurality of different relatively small regions, “microspots”, of a sensor surface a different probe material for interaction with a target material. Thus, for example an SPR probe layer can be configured for assaying a relatively large plurality of different analytes or for characterizing a relatively large plurality of interactions. As a result, SPR sensors and methods are finding increasing use in biochemical applications and SPR sensors and methods are used to identify and assay biomolecules and characterize reactions between biomolecules.
An article by Charles E. H. Berger et al. entitled “Surface Plasmon Resonance Multisensing”, Anal. Chem. Vol. 70, February 1998, pp 703-706, the disclosure of which is incorporated herein by reference, describes an SPR sensor and method that are used to characterize binding of antigens to antibodies. The SPR sensor has a gold SPR conductor formed on a surface, i.e. a sensor surface, of a glass plate, which is optically coupled to a prism. A flow cell comprising four parallel linear “microchannels” (generally, flow channels having at least one dimension about equal to or smaller than a millimeter), each 1 mm wide, 10 mm long and about 0.1 mm deep, is positioned over the SPR conductor. A different antibody is pumped through each microchannel and adsorbed on the gold conductor to form a probe layer. The resulting multi-analyte probe layer comprises a linear array of four different antibodies, each immobilized in a different “antibody” strip on the SPR conductor.
The flow cell is then repositioned so that the microchannels are perpendicular to the antibody strips. A different antigen is pumped through each of the microchannels. Each of the antigens thus comes into contact with each of the four antibodies adsorbed onto the gold conductor. To an extent that the antigen binds with a particular one of the antibodies, it changes an optical property of a region of the antibody strip on which the particular antibody that contacts the antigen is located. Rates at which each antigen of the four antigens binds to each of the four antibodies are determined from measurements of changes in reflectance for light incident on the sensor surface at an angle near to an SPR resonance angle. The article notes that whereas the probe layer was formed by flowing antibodies through microchannels, other methods for forming the probe layer, such as by depositing small quantities of antigen in specific locations using an ink jet nozzle, may be used.
PCT publication WO 02/055993, the disclosure of which is incorporated herein by reference, notes that “electrostatic fields can be used for controlling the extent of immobilization or attachment of biomolecules, such as thiol-derivitized oligonucleotides”, to a surface. The book “Microarray Analysis”, by Mark Schena, John Wiley and Sons, Inc. 2003, the disclosure of which is incorporated herein by reference, describes various methods for depositing or creating small quantities of desired ligands in microspots on a surface to manufacture microarrays. Among the methods described, for example in chapter seven of the book, are contact and non-contact printing methods and photolithographic methods.
U.S. Pat. No. 5,313,264, the disclosure of which is incorporated herein by reference, describes an SPR sensor having a “liquid handling block” comprising a network of microconduits and valves. The network of microconduits and valves is used for moving suitable liquids containing probe material across an SPR conductor formed on a sensor surface so as to generate a probe layer on the SPR conductor and subsequently for moving a target liquid over the probe layer.
The SPR sensor also comprises a substantially monochromatic light source and an optical system for generating a wedge-shaped converging beam from light provided by the light source and directing the wedge-beam onto the sensor surface. The wedge-beam illuminates the probe layer along a spatially fixed, relatively narrow strip-shaped region of the sensor surface with light that is simultaneously incident on the region in a range of incident angles. The range of incident angles is determined by an angle of convergence of the wedge-beam. Light reflected from the sensor surface is imaged on a “two dimensional photodetector device”. Signals provided by the photodetector device are processed to provide a measurement of a change in the refractive index of the probe layer due to interaction of material in the probe layer with material in a target solution that is transported along the probe layer by the liquid handling block.
Many conventional SPR methods and apparatus for forming probe layers, flowing liquids over probe layer surfaces and optically scanning sensor surfaces are relatively complicated, expensive and/or time consuming. Alternative SPR sensors and methods for generating multi-analyte probe layers, pumping liquids over probe layers and illuminating sensor surfaces are needed.